This hilarious poem which is incredibly popular in Britain is above all an ironic reflection about aging and the way that when we are old it is permissible for us to act in strange ways and do things that "normal" people would never ever dream of doing. The label of being "old" clearly gives us license to be slightly crazy, and, as the poem says, "wear purple" and other colours that clash with it deliberately. Note how the speaker comments upon not being old yet and what is expected of you:
But now we must have clothes that keep us dry
And pay our rent and not swear in the street
And set a good example for the children.
We must have friends to dinner and read the papers.
Clearly, not being old is presented as being very boring as we have to follow the norms and values of society and are unable to do crazy things without censure. The final stanza of this brilliant poem playfully asks whether the speaker should actually start acting crazily now so that people are not so surprised and shocked when suddenly old age descends and she starts wearing purple.
This delightful poem therefore is a playful discussion of old age and the way that society treats those who are old compared to those that aren't.